U.S. patent application number 14/493274 was filed with the patent office on 2016-03-24 for single blade propeller with variable pitch.
The applicant listed for this patent is The Boeing Company. Invention is credited to Blaine K. Rawdon.
Application Number | 20160083077 14/493274 |
Document ID | / |
Family ID | 55525048 |
Filed Date | 2016-03-24 |
United States Patent
Application |
20160083077 |
Kind Code |
A1 |
Rawdon; Blaine K. |
March 24, 2016 |
SINGLE BLADE PROPELLER WITH VARIABLE PITCH
Abstract
An improved performance propeller employs a single propeller
blade having an axis of rotation and a centripetal force about the
axis. A pitch control unit is mounted opposite the single propeller
blade and has a compensating centripetal force with respect to the
single propeller blade about the axis..
Inventors: |
Rawdon; Blaine K.; (San
Pedro, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
The Boeing Company |
Chicago |
IL |
US |
|
|
Family ID: |
55525048 |
Appl. No.: |
14/493274 |
Filed: |
September 22, 2014 |
Current U.S.
Class: |
416/148 ;
29/889.6; 416/147; 416/153 |
Current CPC
Class: |
B64C 11/00 20130101;
B64C 11/44 20130101; B64C 11/06 20130101 |
International
Class: |
B64C 11/44 20060101
B64C011/44; B64C 11/30 20060101 B64C011/30 |
Claims
1. An improved performance propeller comprising: a single propeller
blade having an axis of rotation and a centripetal force about said
axis; a pitch control unit mounted opposite the single propeller
blade and having compensating centripetal force with respect to the
single propeller blade about said axis.
2. The improved performance propeller as defined in claim 1 wherein
the pitch control unit comprises: a gear box connected to a hub
tube of the single propeller blade, said gear box rotating said hub
tube about a blade axis to define pitch of the single propeller
blade; and, a pitch motor operably connected to the gear box.
3. The improved performance propeller as defined in claim 2 further
comprising: at least one bearing engaging the hub tube to support
rotation of the hub tube; and, at least one retention device fixing
said hub tube radially with respect to the axis of rotation.
4. The improved performance propeller as defined in claim 3 wherein
the hub tube extends substantially perpendicularly through a
propeller shaft and further comprising a drive motor connected to
rotationally power said propeller shaft and, said at least one
bearing comprises a pair of bearings diametrically opposed across
the axis of rotation supporting said hub tube in said propeller
shaft; and, said at least one retention device comprises a first
lock ring engaging the hub tube adjacent a first one of the pair of
bearings and a second lock ring engaging the hub tube adjacent a
second one of the pair of bearings.
5. The improved performance propeller as defined in claim 4 further
comprising a coupler adapted to engage one of said lock rings to a
rotary actuator shaft extending from the gear box.
6. The improved performance propeller as defined in claim 2 wherein
the blade axis has a rake angle with respect to the rotational
axis.
7. The improved performance propeller as defined in claim 2 wherein
at least one of said gear box and pitch motor are offset from the
blade axis.
8. The improved performance propeller as defined in claim 3 wherein
the hub tube extends through a cross-member supported by a yoke,
said cross member rotatable within the yoke allowing said single
propeller blade and pitch control unit to teeter about a teeter
axis.
9. The improved performance propeller as defined in claim 8 wherein
the teeter axis is angled with respect to the rotational axis.
10. The improved performance propeller as defined in claim 2
further comprising a controller operably connected to the pitch
motor and an electrical power source and receiving instructions for
blade pitch, said controller providing electrical power to said
pitch motor responsive to said instructions.
11. The improved performance propeller as defined in claim 10
wherein the power source is an airplane power source and
commutating slip rings and brushes connect the power source to the
controller.
12. The improved performance propeller as defined in claim 10
wherein the power source is a generator, said generator having at
least one magnet mounted on a stationary side of an interface
between stationary and rotating components and at least one coil
mounted adjacent to and rotating with the pitch control unit, said
coils connected to the controller to provide electrical power.
13. The improved performance propeller as defined in claim 12
further comprising a battery connected to the coils through the
controller, said controller distributing power to the battery.
14. The improved performance propeller as defined in claim 10
further comprising a receiver connected to the controller to
receive said instructions.
15. The improved performance propeller as defined in claim 14
further comprising a position sensor in the pitch control unit,
said position sensor providing a pitch angle output to the
controller.
16. The improved performance propeller as defined in claim 15
further comprising a transmitter connected to the controller and
transmitting said pitch angle output.
17. The improved performance propeller as defined in claim 2
further comprising a brake connected to a selected one of the pitch
motor and gearbox.
18. A method for counter balancing a single blade propeller for
higher efficiency in thrust production over conventional
multibladed propellers , said method comprising: selecting a single
propeller blade with radius and chord profile to provide a higher
efficiency than a baseline multibladed propeller; mounting the
propeller blade with a hub shaft supported by bearings to a
propeller shaft extending from a motor; securing the hub shaft with
retention devices ; attaching a pitch control unit to the propeller
shaft with a support bracket and to the hub shaft extending
oppositely from the single propeller blade; and, spacing a center
of gravity of the pitch control unit relative to an axis of
rotation of the propeller to provide a balancing force for the
propeller blade.
19. The method as defined in claim 18 further comprising: attaching
a pitch control motor to drive a gearbox having a rotary actuator
output shaft engaging the hub shaft for pitch control;
20. The method as defined in claim 19 further comprising: providing
electrical power for the pitch control motor by connecting an
airplane power source through a commutator and slip ring
arrangement from a stationary side of a nacelle to a rotating side
of a propeller hub.
21. The method as defined in claim 10 further comprising: providing
electrical power for the pitch control motor by a generator having
at least one magnet on a stationary side and at least one coil on a
rotating side..
22. The method as defined in claim 19 further comprising: providing
power with a controller to the pitch control motor and to a
chargeable energy storage element.
23. The method as defined in claim 22 further comprising:
monitoring blade pitch with a position sensor; and transmitting
said blade pitch to an airplane controller.
24. The method as defined in claim 18 further comprising: employing
a teetering yoke to attach the pitch control unit and hub shaft to
the propeller shaft.
25. The method as defined in claim 18 further comprising: angularly
mounting the propeller blade and pitch control unit to create a
blade rake angle.
Description
BACKGROUND INFORMATION
[0001] 1. Field
[0002] Embodiments of the disclosure relate generally to the field
of propulsion systems for aircraft and more particularly to a
single blade propeller with variable pitch employing an
electrically powered pitch control mechanism as a
counterweight.
[0003] 2. Background
[0004] Solar powered airplanes are typically powered by electrical
motors receiving power from a solar array on the surface of the
aircraft and driving multiple bladed propellers. In most cases
these airplanes are designed for very high altitude flight with
long duration flight profiles. Performance of such solar powered
airplanes is very sensitive to component efficiency. Propeller
efficiency is approximately equivalent in importance to airframe
lift to drag (L/D). In example prior art systems a 2.0% absolute
efficiency improvement in propeller efficiency may offset an
airplane weight increase of 1.3% or increase winter solstice
maximum latitude by approximately 1.degree..
[0005] Additionally, such solar powered airplanes have little
reserve power for climb, even at low altitude. In dense low
altitude air at slow flight speeds, the motor bogs down at low
propeller rotational speed (RPM). Even though the airplane requires
far less power to fly, the motor is current-limited by overheating
concerns and produces little excess power.
[0006] It is therefore desirable to provide a means to increase
motor RPM to increase motor power without exceeding the current
limit.
SUMMARY
[0007] Exemplary embodiments provide an improved performance
propeller employing a single propeller blade having an axis of
rotation and a centripetal force about the axis. A pitch control
unit is mounted opposite the single propeller blade and has a
compensating centripetal force with respect to the single propeller
blade about the axis.
[0008] The embodiments disclosed provide a method wherein a single
blade propeller is counter balanced for higher efficiency in thrust
production over conventional multibladed propellers by selecting a
single propeller blade with radius and chord profile to provide a
higher efficiency than a baseline multibladed propeller. The
propeller blade is mounted with a hub shaft supported by bearings
to a propeller shaft extending from a motor and secured with lock
rings. A pitch control unit is attached to the propeller shaft with
a support bracket and to the hub shaft extending oppositely from
the single propeller blade. The center of gravity of the pitch
control unit is spaced relative to an axis of rotation of the
propeller to provide a balancing force for the propeller blade.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] The features and advantages of embodiments disclosed herein
will be better understood by reference to the following detailed
description when considered in connection with the accompanying
drawings wherein:
[0010] FIG. 1 is perspective view of an exemplary airplane in which
the present embodiments may be employed;
[0011] FIG. 2 is a section view showing the components of the
embodiment;
[0012] FIG. 3A is a side detailed section view of the pitch control
unit and propeller blade hub;
[0013] FIG. 3B is a front detailed section view of the pitch
control unit and propeller blade hub;
[0014] FIG. 3C is a side detailed view of an alternative
configuration of the blade rake angle;
[0015] FIG. 3D is a side detailed view of an alternative
configuration of the pitch control unit elements to counter offset
thrust;
[0016] FIG. 3E is a front detailed view of an alternative
configuration of the pitch control unit elements to counter offset
torque
[0017] FIG. 4 is a side section view of a second embodiment of the
pitch control unit;
[0018] FIG. 5 is a perspective view of a spinner with stub fairing
for use with the embodiments described;
[0019] FIG. 6 is a perspective view of a blade teetering
structure;
[0020] FIG. 7 is an overlay of a first single blade propeller blade
arrangement with double the chord compared to a baseline two blade
propeller;
[0021] FIG. 8 is an overlay of a second single blade propeller
blade arrangement with double the radius compared to a baseline two
blade propeller;
[0022] FIG. 9 is an overlay of a third single blade propeller blade
arrangement with proportionally increased radius and chord compared
to a baseline two blade propeller; and,
[0023] FIGS. 10A and 10B are a flow chart of a method for
implementation of the embodiment described herein.
DETAILED DESCRIPTION
[0024] An exemplary embodiment disclosed herein provides a variable
pitch, single-blade propeller balanced by a pitch control mechanism
providing improved performance over a baseline multibladed
propeller. An alternative embodiment includes blade teeter. While
disclosed herein with respect to a solar powered airplane for high
altitude flight, the embodiment is applicable to alternative
airplane types and missions.
[0025] Employing a variable pitch propeller allows the single blade
to be varied in pitch angle to permit higher motor RPM at low
airplane true airspeed, increasing motor power and airplane climb
rate Improved low altitude climb rate may permit the propeller and
motor design to be optimized for the high altitude condition,
improving performance in the dominant flight condition.
[0026] However, the variable pitch mechanism additionally serves as
counterweight for the blade since propellers must be balanced about
the propeller axis and single-blade propellers are inherently
imbalanced and require the addition of a counterweight. Use of the
variable pitch mechanism provides needed blade counterweight
reducing or eliminating the need for "dead" counterweight material.
The variable pitch mechanism may be electrically powered and
controlled. Electrical power and control is already available on a
high altitude, low power airplane such as a solar powered airplane.
In alternative embodiments, the blade pitch mechanism may be
electrically powered but signaled by radio for pitch angle control.
It is desirable that the variable pitch mechanism may fail without
a change in blade pitch. The mechanism for the embodiments
disclosed may be designed with a brake or with irreversible gears
(such as a worm gear) so that a loss of power to the mechanism does
not result in a change in propeller blade pitch. Once the airplane
is at its design altitude and speed, changes in propeller pitch may
not be needed and a failure of the mechanism may not influence
flight performance or endurance.
[0027] It may not be optimal to have the propeller axis
perpendicular to the propeller shaft axis. Furthermore, it may not
be optimal to provide a dynamically balanced system. The blade cone
angle and the propeller/pitch actuation system center of gravity
may be determined to diminish net forces resulting from blade
thrust and torque reaction offset from the propeller axis.
Additionally, blade teeter maybe employed as a means to eliminate
moments resulting from the single blade's offset thrust line (with
respect to the propeller shaft axis). Blade teeter may also improve
the propeller disk loading uniformity as well as blade lift
coefficient uniformity as it sweeps the propeller disk.
[0028] The single propeller blade may be characterized by a larger
chord than a multi-blade design. This increases the Reynolds number
on the blade and improves blade element L/D, which may improve
propeller performance In addition, the blade may optimally sweep a
larger diameter than an equivalent multi-blade propeller to reduce
propeller induced losses. A single blade propeller employed on
multiple wing mounted electric motors sheds approximately one-half
as many wakes across the wing thereby reducing losses associated
with the wake. The single blade is additionally likely to optimize
at a higher segment lift coefficient than an equivalent multi-blade
prop, which may provide better off-design performance The single
blade may be retained in a propeller hub with an external fitting
that may be lightweight, reliable and easily inspected and
repaired.
[0029] Referring to the drawings, an exemplary solar powered
aircraft concept on which the present embodiments may be employed
is shown in FIG. 1. The aircraft 10 incorporates a fuselage 12 with
wings 14a and 14b. An empennage employs a vertical stabilizer and
rudder 16 and horizontal stabilizers with elevators 18a, 18b. While
shown as a conventional aircraft layout, alternatives may employ
flying wing or canard designs. A solar array 20 arranged over the
upper surfaces of the wings and other surfaces of the aircraft
provides electrical power. Multiple motor pods 22a-22f support
electric motors driving propellers (not shown).
[0030] FIG. 2 shows an exemplary motor pod 22 employing the present
embodiment. A fairing or nacelle 24 encloses the motor 26 and
provides mounting structure for attachment to the aircraft wing or
other supporting structure. A propeller blade 28 is mounted to a
drive shaft 30 extending from the motor 26 with a hub tube 32
extending through the shaft. A gear box may be employed between the
motor 26 and drive shaft 30 to achieve desired RPM. A pitch control
unit 34 attaches to the hub tube 32 on the opposite side of the
shaft 30 from the propeller blade 28. The mass of the pitch control
unit and its center of gravity 35 are positioned to provide a
balancing force for the propeller blade 28 and its center of
gravity 29 about an axis of rotation, which for the embodiment
shown is a propeller drive shaft axis 42. The pitch control unit 34
incorporates a gearbox 36 and a pitch control motor 38 attached to
and driving the gearbox. Operation of the gearbox 36 rotates the
hub tube 32 which rotates the propeller blade 28 about blade axis
40 extending approximately radially outward from a center of the
propeller, approximately in the plane of rotation of the propeller
and is substantially perpendicular to the propeller drive shaft
axis 42. The propeller drive shaft axis 42 also constitutes the
axis of rotation of the propeller. Rotation of the propeller blade
28 about the blade axis 40 constitutes variable pitch.
[0031] For descriptive purposes herein, blade pitch is said to
increase when the blade leading edge is rotated forward (where
forward is the general direction of flight). Decreased blade pitch
is the opposite. Shaft power provided by the motor 26 is the
product of torque and RPM. Electric motor torque is approximately
proportional to motor current. Motor current may be limited by
cooling capacity. This means that motor torque is limited so
maximum motor power is proportional to RPM. Propeller thrust is the
product of shaft power times propeller efficiency divided by true
airspeed. An increase in blade pitch tends to increase the lift
coefficient of each blade section. If RPM is held constant this
results in increased shaft torque and a greater power requirement
from motor and increased thrust. If power remains constant, the
result will be reduced RPM. If torque remains constant, RPM will be
reduced and power will drop. When propeller pitch and RPM are held
constant, an increase in airspeed tends to reduce blade section
lift coefficient, thereby reducing shaft torque and thrust.
Conversely, a decrease in airspeed tends to increase blade section
lift coefficient, increasing torque and thrust. If constant torque
is applied with variations in airspeed, the propeller RPM will vary
approximately as true airspeed and shaft power declines
approximately in proportion to true airspeed. Propeller efficiency
varies with advance ratio which may be defined as propeller tip
circumferential speed divided by true forward airspeed. Tip
circumferential speed is proportional to RPM. For each propeller
design, there is a single advance ratio that provides maximum
efficiency. A long-endurance airplane may optimize its propeller
design to provide needed thrust at this most-efficient advance
ratio.
[0032] For the embodiment shown in FIG. 2, electrical power and the
control signal for the pitch control motor 38 are provided with
slip rings 44 and brushes 46 electrically interconnecting power and
signal conductors 48 across the rotating shaft 30 from the airplane
power and control system, generally represented as block 49. For
the embodiment shown, three wires are employed, positive and
negative for power and a third wire with an encoded signal. The
encoded signal provides instructions to define a target pitch
position for the pitch control mechanism which moves under power to
that target and then holds that position
[0033] Details of the pitch control unit and propeller blade
mounting are seen in FIGS. 3A and 3B (individual electrical wire
connections are not shown for clarity). The propeller drive shaft
incorporates one or more bearings 50 the axes of which are
perpendicular to the propeller shaft and co-linear. For the
embodiment shown a pair of bearings 50 are employed diametrically
opposed across the propeller shaft axis 42. The bearings 50 receive
the propeller blade hub tube 32 and permit the hub tube and
attached propeller blade 28 to rotate about its axis 40 (change
pitch as previously described). The bearings 50 transfer blade
loads into the propeller shaft. Optionally the hub tube 32 may be
conical, or stepped but in any case is circular in a cross section
taken perpendicular to the axis 40 where the hub tube fits into the
bearings 50 and through a bearing bore 52. The hub tube 32 is
approximately aligned with the radial axis 40 of the propeller
blade 28. The hub tube has features that enable two functions;
retention of the propeller blade 28 along its axis in the outward
direction and connection to the blade pitch control unit 34. Lock
rings 54a and 54b or a similar retention device limits how far the
hub tube goes into the propeller shaft bearings on both sides of
the drive shaft and secures the hub shaft and blade to the
propeller shaft 30.
[0034] The blade pitch control unit 34 controls blade pitch angle
in response to signals originating elsewhere (e.g. the airplane's
flight control computer). A rotary actuator output shaft 56
extending from the gearbox 36 attaches to the propeller blade hub
tube 32. For the embodiment shown in the drawings, a coupler 58
engages the lock ring 54b. The actuator output shaft is fixedly
mounted to the propeller through the hub tube to resist torsional
forces about the propeller radial axis 40 transmitted by the
propeller blade.
[0035] For the embodiment shown in the drawings, the pitch control
unit 34 is an electro-mechanical device employing the electric
pitch control motor 38 which may be favorably a stepper motor,
however, other motor types are feasible. A motor controller 60 may
be used to operate the pitch control motor 38 in response to
supplied control signals when supplied with power. The gearbox 36
may favorably provide an extreme gear reduction from the motor to
the rotary actuator output shaft 56 and hub tube coupler 58. This
reduces required motor torque and increases blade pitch control
accuracy. A position sensor 62 may be included in association with
the gearbox to provide a pitch angle output for feedback to a
flight control computer regarding the propeller's pitch angle.
Alternatively the flight control computer may deduce the propeller
pitch angle from motor current, motor RPM, airspeed, altitude and
other available data.
[0036] The pitch control unit 34 acts as a counter-weight for the
one-bladed propeller. The actuating elements of the pitch control
unit 34, the gearbox 36 and pitch control motor 38, are mounted to
the propeller shaft 30 on the opposite side from the single
propeller blade 28 with a support frame 64. The characteristics of
the actuating elements are designed to balance the propeller blade.
The weight, distance of the center of gravity from the propeller
shaft axis 42 and distance of the center of gravity from the
propeller radial axis 40 may be adjusted to provide the desired
balance.
[0037] The net thrust of a single-blade propeller is offset from
the propeller shaft. This creates a moment on the propeller shaft
30 and the supporting structure for the motor 26. It may be
beneficial to rake the propeller axis 40 forward with respect to a
plane perpendicular to the propeller shaft axis 42. In such a
configuration the propeller blade sweeps a cone instead of a plane
or disk. The angle at which the blade is raked forward may be
called the "cone angle". Rotation of the propeller creates a
centripetal force on the blade in an outward, radial direction.
With the center of gravity of the propeller ahead of the propeller
root, a moment is created that offsets the moment created by the
offset thrust. The propeller rake may be adjusted to minimize the
total moment at a particular flight condition by angularly mounting
the pitch control unit and single propeller blade non-perpendicular
with a rake angle 65 to the propeller shaft axis 42 as shown in
FIG. 3C.
[0038] Instead of or in addition to raking the propeller forward,
it may also be beneficial to rake or offset the center of gravity
of the pitch control unit for purposes of countering offset thrust
or offset torque. Offset thrust may be accommodated, in addition to
or alternative to modifying the coning angle described above, by
designing the gear set in the gearbox 36 to offset the rotary
actuator output shaft 56' as shown in FIG. 3D to move the center of
gravity of the gear box and motor aft.
[0039] The net circumferential force of a single-blade propeller
(due to its reaction of propeller shaft torque) is offset from the
propeller shaft axis (offset torque). This creates an equal but
opposite force on the propeller shaft that is perpendicular to both
the propeller shaft axis and the propeller blade axis. It may be
beneficial to offset this force at a particular flight condition by
offsetting the mass of the pitch control unit in a direction
perpendicular to a plane defined by the propeller blade axis and
the propeller shaft axis in the direction of prop rotation. This
offset moves the center of gravity of the entire propeller system
to a point that is offset from the plane defined by the propeller
axis and propeller shaft. This may be accomplished by designing the
gear set in the gearbox 36 to offset the rotary actuator output
shaft 56' as shown in FIG. 3E. This shifts the mass of both the
gearbox 36 and pitch control motor 38 out of the propeller blade
axis--propeller shaft axis plane in a direction toward the blade
leading edge 31 as seen from the front. The resulting centripetal
force may tend to offset the torque reaction force.
[0040] Power produced by the airplane may be transmitted to the
pitch control unit via wires and commutation (conductive rings and
brushes) at the interface between the stationary and rotating
components as previously described with respect to FIG. 2.
Alternatively, power may be generated at an interface between the
stationary (drive motor and nacelle) and rotating components
(propeller drive shaft, propeller hub and pitch control unit, and a
spinner as seen in FIGS. 2 and 5) with a generator 65 as shown in
FIG. 4. A ring of one or more magnets 66 may be mounted to the
stationary side of the interface. A series of one or more wire
coils 68 can be arranged in a ring mounted to the propeller side.
These coils can be connected to the controller 60. When the
propeller shaft 30 spins, the coils move past the magnets and
generate electricity which is rectified and otherwise processed by
the controller 60 into a form usable by the pitch control motor 38
or an energy storage system. The controller 60 provides power to
the pitch control motor 38 and can regulate power generation to
maintain a favorable charge state in a battery 70, or alternatively
a capacitor, which may store electrical energy. Stored electrical
energy may provide extra power for actuation loads or signal
receiver power.
[0041] The controller 60 receives control signals by hard wire via
commutation (conductive rings and brushes) as previously described
for connecting to the flight control computer or other control
device in the airplane providing an encoded signal or other form of
instructions for pitch control. Alternatively, electromagnetic
transmission (e.g. radio) can be employed with a radio
transmitter/receiver 72 as part of the pitch control unit to
receive the pitch control instructions. While shown in the
embodiment as a single combined unit, the radio transmitter may be
part of the airplane system connected to the flight control
computer or other control device and may transmit pitch position
signals of the pitch control unit 34, for example the pitch angle
output from the position sensor 62. The pitch control unit may
thereby inform the airplane flight control computer of the
propeller pitch position and other predetermined operational
data.
[0042] As further shown in FIG. 4, the pitch control unit 34 may
fix (lock) the blade pitch in the absence of electrical power or
control signal. This capability gives "fail-safe" operation if
failure occurs at an on-design operating point. Since 99% of flight
time is spent on-design, likelihood of a failure occurring in the
on-design condition is good. Blade pitch locking may be achieved
using an irreversible gearbox, for example, a worm gear system
which cannot be back-driven. Alternatively, an active or passive
brake 74 may be employed. The brake may incorporate a mechanism
that engages one or more gear teeth or a mechanism that creates
friction when the motor is not running.
[0043] The pitch control unit 34 may additionally provide a means
for fine-tuning propeller balance. An additional component (such as
a ballast weight) may be adjusted in radial position to fine-tune
propeller balance. Alternatively, location of one or more
components of the actuator system may be adjustable within the
support frame 64 along the propeller's radial axis to fine-tune
propeller balance.
[0044] As shown in FIG. 5, a propeller spinner 74 is provided as an
aerodynamic fairing that encloses the propeller hub including the
hub tube 32, bearings 50 and lock rings 54a, 54b and some portion
of the propeller pitch control unit 34 seen in FIGS. 3A-3C and 4.
An aft edge 76 of the spinner 74 is coordinated with the fairing or
nacelle 24 that encloses the motor 26 and motor support as shown in
and described with respect to FIG. 2. There may be some provision
for cooling air entry into the nacelle to cool the motor. The
spinner 74 is generally axially symmetric and has an opening for
the propeller that permits the propeller to change pitch without
conflicting with the spinner. It may also have a streamlined stub
fairing 78 that encloses the portion of the pitch control unit 34
that extends beyond the axially symmetric surface of the spinner.
Lighter pitch control unit elements can balance the propeller blade
if the pitch control unit projects farther from the propeller shaft
axis 42 with only a small aerodynamic penalty of the fairing. The
stub fairing 78 may be aligned with the local flow in the design
condition. The stub fairing 78 may have an incidence angle that
aligns with the approximately helical flow produced by the
propeller blade 28.
[0045] A blade teeter mechanism, shown in FIG. 6, may be employed
to enable the propeller blade 28 to swing approximately forward and
aft, approximately in the plane defined by the propeller blade axis
40 and the propeller shaft axis 42. The embodiment shown is but one
example. A yoke 80 interconnects the propeller shaft 30 with the
pitch actuation unit support frame 64 and the blade 28 and pitch
control unit 34 are free to swing fore and aft about a teeter axis
82 which is substantially perpendicular to the propeller shaft axis
42. The blade 28, as before, is able to rotate about blade axis 40
within bearings 50 now supported in a cross-member 84 that spans
the yoke 80 for change of pitch. Bearings 86 in the yoke permit
rotation of the cross-member 84, the propeller blade 28 and pitch
control unit 34.
[0046] Two benefits accrue from a teetering blade. First, the
moment on the propeller shaft 30 resulting from the offset of the
single propeller blade 28 net thrust is eliminated. Second, the
propeller blade has a natural tendency to temper fluctuations in
blade angle of attack resulting from non-uniform inflow. This can
reduce induced losses and blade parasitic losses. In certain
embodiments, it may be beneficial to angle the teeter axis 82 with
respect to the propeller shaft axis 30 so that a forward motion of
the propeller blade results in a reduced blade angle of attack.
This will increase the natural tendency of the propeller blade to
temper fluctuations in blade angle of attack resulting from
non-uniform inflow.
[0047] To improve the efficiency of the propulsion system but
maintain the same airplane performance, a single blade propeller
must produce the same thrust as an equivalent multi-blade prop. For
the purpose of examples herein, a baseline propeller 100 of two
blades having a defined chord and radius is compared to the single
blade propeller 28 of the embodiments described. If the blades
operate at the same lift coefficient the single blade can have the
same radius to provide an equal diameter of revolution 102 but a
chord 104 approximately twice as wide as the baseline propeller 100
to achieve the necessary thrust performance. This propeller will
have reduced parasitic losses due to the increased Reynolds number
of the blade sections and will have approximately the same induced
losses because its diameter and thrust are unchanged. There may be
a minor increase in induced losses due to greater irregularity of
the disk loading from the single blade. The single bladed propeller
28a in this configuration is illustrated in FIG. 7 compared to the
baseline two bladed propeller 100.
[0048] Alternatively, the single blade can have approximately the
same chord as the blades of the baseline two bladed propeller but
approximately twice the radius providing a diameter of revolution
106 twice the size of the baseline propeller. This propeller will
operate at about half the RPM of the two-blade propeller and will
the same blade efficiency (blade section L/D) but have reduced
induced losses. The single bladed propeller 28b in this
configuration is illustrated in FIG. 8 compared to the baseline two
bladed propeller 100.
[0049] As a second alternative, a single propeller blade 28c can
have some combination of radius and chord that provides
approximately the same total blade area as the equivalent two-blade
prop. As an example the radius is about midway between the two
cases above (i.e. 1.41 times the radius of the two-blade propeller)
providing a diameter of revolution 110 and the span to chord
proportion of the blade are similar increased by a factor of 1.41
as exemplified by chord 108. This propeller will operate at about
2/3 the RPM of the baseline two-blade propeller and, relative to
the two-blade propeller, this single propeller blade may have
reduced induced losses and improved blade section L/D. The single
bladed propeller 28c in this configuration is illustrated in FIG. 9
compared to the baseline two bladed propeller 100.
[0050] A method for counter balancing a single blade propeller for
higher efficiency in thrust production over conventional
multibladed propellers is shown in FIGS. 10A and 10B. A single
propeller blade is selected with radius and chord profile to
provide a higher efficiency than a baseline multibladed propeller,
step 1002. The propeller blade is mounted with a hub shaft
supported by bearings to a propeller shaft extending from a motor,
step 1004 in a stationary nacelle. The hub shaft is secured with
retention devices such as lock rings, step 1006. A pitch control
unit is attached to the propeller shaft with a support bracket,
step 1008, and to the hub shaft extending oppositely from the
single propeller blade, step 1010. The pitch control unit is spaced
with its center of gravity relative to an axis of rotation of the
propeller to provide a balancing force for the propeller blade,
step 1011. A pitch control motor is attached to drive a gearbox,
step 1012 having a rotary actuator output shaft engaging the hub
shaft for pitch control, step 1014. Electrical power is provided
for the pitch control motor by connecting an airplane power source
through a commutator and slip ring arrangement from the stationary
side of the nacelle to the rotating side of the propeller hub or by
a generator having one or more magnets on the stationary side and
one or more coils on the rotating side, step 1016. A controller
provides power to the pitch control motor, step 1018 and to a
chargeable energy storage element such as a battery or capacitor,
step 1020. Blade pitch is monitored with a position sensor, step
1022, and transmitted to an airplane controller through the
commutator and slip ring system or by a transmitter/receiver in the
pitch control unit, step 1024. A teetering yoke may be employed to
attach the pitch control unit and blade hub to the propeller shaft,
step 1026, to allow the single propeller blade to teeter.
Alternatively a blade rake angle may be created by angularly
mounting the propeller blade and pitch control unit, step 1028.
[0051] Having now described various embodiments in detail as
required by the patent statutes, those skilled in the art will
recognize modifications and substitutions to the specific
embodiments disclosed herein. Such modifications are within the
scope and intent of the present disclosure as defined in the
following claims.
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